Many measurement instruments include a display device, and/or provide outputs for a display device, for displaying waveform images corresponding to the instrument's measurement of an input signal applied to the instrument. Examples of such instruments include oscilloscopes, logic analyzers and digital spectrum analyzers. In these instruments, waveform images are drawn onto a rasterized display device at the display device's video frame rate—for example, 60 frames per second.
The “persistence” of a waveform image describes the length of time that the waveform image continues to be displayed in whole or in part on the display device before it is erased. If the waveform image is erased one frame after it is drawn, then the waveform image has a minimum persistence (also sometimes referred to as non-persistence). At the other extreme, if the waveform image was never erased from the display device, then the waveform image would have infinite persistence. Between these two extremes lies a range of finite persistence for a displayed waveform image.
FIG. 1 illustrates an example oscilloscope 100 with a variable persistence. Oscilloscope 100 includes inputs 110 for receiving signals, a display device 120 for displaying one or more waveform images corresponding to the instrument's measurement of one or more input signals, and a control section 130 having a plurality of control inputs, including variable persistence control inputs 135 (e.g., rotatable knobs), one for each channel of the oscilloscope.
FIG. 2 shows a functional block diagram of portions of one example of a digital oscilloscope 200. Oscilloscope 200 includes probe 210, a vertical system 220, a trigger system 230, a horizontal system 240, an acquisition system 250, a digital display system 260, and a display device 270.
Often it is desirable to give a user of a measurement instrument one or more control inputs (e.g., variable persistence control inputs 135 in oscilloscope 100) for receiving a user input for controlling or adjusting the persistence of displayed waveform images. When the user selects a small persistence value (e.g., 0.5 seconds), then the waveform image remains displayed for only a short amount of time, providing a lively, responsive display. Longer persistence times allow the waveform images to linger on the display, allowing a user more time to analyze the waveform.
In existing instruments, this flexibility comes at a price in terms of memory requirements imposed on the instrument. The common approach to displaying waveforms for a variable amount of time on a display device in a digital oscilloscope is to keep track of how long each pixel of the waveform image has been displayed and to decrement this time to zero at a regular rate. One common implementation stores in memory a “time-left-on-screen” value for each pixel of the display device. For each video frame, this value is read from memory for each pixel. If the value is non-zero, then the pixel is displayed, the value is decremented, and the new value is written back into memory. This is repeated for every channel of the oscilloscope for which a persistent waveform image is displayed. This requires a high-speed multiple-bit decrementer for each channel. This also requires a read-modify-write memory operation for each pixel in each video frame. With a display device having a large number of pixels, this translates to a large amount of display memory with a high display memory bandwidth, resulting in relatively expensive display memory system.
It would be desirable to provide another solution for displaying a waveform with variable persistence in display device associated with a measurement instrument that could be less complex and less expensive.